A) Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a semiconductor device with copper wirings.
B) Description of the Related Art
In order to realize high speed operation and low power consumption of LSI circuits, device elements are made fine, and drive voltage and resistance of wiring are reduced. In order to lower the resistance of wiring, Cu wiring having a lower resistivity than conventionally used Al wiring is adopted. Fine patterning of a Cu layer is difficult so that a damascene structure is often adopted. For fine device elements and low power consumption, it is desired to realize a high speed operation of transistor at a low voltage. To this end, a gate insulating film is made substantially thin.
A diffusion speed of Cu in a silicon oxide film as an interlayer insulating film is very high. As Cu diffuses in the interlayer insulating film, the transistor characteristics are degraded. If Cu is used as the material of wiring, it is necessary to dispose a diffusion prevention film around Cu wiring. An oxide film hardly suppresses Cu diffusion.
A Cu wiring layer is deposited in a trench (and/or a via hole) formed through a silicon oxide film and exposing a lower wiring layer or lower conductive layer. In this case, a barrier metal layer and a CU wiring layer are stacked, the barrier metal layer having a function of preventing Cu diffusion. Since the barrier metal layer is disposed between the Cu wiring layer and silicon oxide film, Cu can be prevented from being diffused into the silicon oxide film. The Cu wiring layer deposited on the silicon oxide film is removed by chemical mechanical polishing (CMP) or the like. If an upper silicon oxide layer is deposited on this substrate, Cu of the exposed wiring layer diffuses into the upper silicon oxide layer. A silicon nitride film as an insulating member having a function of Cu diffusion prevention is formed covering the damascene Cu wiring layer.
A silicon oxide film is widely used as a gate insulating film. If a gate oxide film is thinned, direct tunneling current flowing through the gate oxide film increases, i.e., gate leak current increases. In order to reduce the gate leak current, a physical thickness of the gate insulating film is increased.
In order to thicken the gate insulating film and realize a transistor drive capability similar to a thinned gate insulating film, it is necessary to use a high dielectric constant insulator as the material of a gate insulating film. As the dielectric constant of a gate insulating film is made high, it is possible to increase the physical thickness of the film and form an electrically thinned gate insulating film. Both suppressing gate leak current and improving a transistor drive ability can therefore be realized.
In a surface channel type p-channel transistor, if boron impurities in the gate electrode diffuse into the gate insulating film and reach the channel region, the transistor characteristics are degraded.
In order to alleviate this problem, technique of doping nitrogen in a gate insulating film has been adopted. If the nitrogen concentration in the gate insulating film is made too high, the nitrogen concentration at the interface between the gate insulating film and semiconductor substrate becomes high so that the motion of electrons/holes flowing in a channel formed in the substrate surface layer is hindered.
The phenomenon that drain current flowing through the transistor is deteriorated is called negative bias temperature instability (NBTI). This deterioration appears as reduced drain current when a p-channel MOS transistor is subjected to an accelerated test at a raised temperature by applying a negative voltage to the gate. Drain current deterioration ΔIds per stress time of copper wiring is larger than aluminum wiring.
Even if a gate insulating film is made thick to lower direct tunneling current, the NBTI deterioration poses a problem. If the nitrogen concentration in a gate insulating film is increased, the NBTI deterioration increases.
Many problems occur if a gate insulating film is made thin in order to improve the operation characteristics of LSI. A novel transistor structure capable of solving these problems has been desired.